In order to vary the load pressure prevailing in the outlet port of such a valve, the piston is subjected to an external control force tending to change the effective width of the restricted passage against the countervailing action of a restoring force. The control force may be transmitted mechanically, e.g. by a cam acting through a coil spring, or electromagnetically, as by a solenoid. In the latter case, e.g. as described in German published specifications Nos. 2,225,305 and 2,353,262, the exerted control force will be substantially proportional to the applied input current; the maintenance of a similar proportionality between that current and the resultant load pressure, however, is more difficult to achieve.